1. Field of the Invention
This invention relates to heat-developable imaging materials and processes for developing an image employing an image-forming combination comprising (i) a tellurium (II) or (IV) compound as an oxidizing agent with (ii) a reducing agent. In one of its aspects, it relates to a heat-developable imaging element comprising an oxidation-reduction image-forming combination comprising (i) a tellurium (II) or (IV) compound as an oxidizing agent with (ii) a reducing agent and physically developable metal nuclei. Another aspect of the invention relates to a heat-developable imaging composition comprising the described image-forming combination.
2. Description of the State of the Art
It is known to provide an image with an imaging material, especially a photographic imaging material by so-called dry processing with heat. These materials are sometimes described as heat-developable photographic materials or photothermographic materials. Such heat-developable photographic materials after imagewise exposure are heated to provide a developed image in the absence of processing solutions or baths.
It has been desirable to provide reduced silver concentrations in heat-developable imaging materials. Attempts have been made in the past to provide this desired reduced silver concentration. For example, U.S. Pat. No. 3,152,903 of Shepard et al., issued Oct. 13, 1964 describes a so-called dry-processable imaging material containing a non-silver material. It is indicated that the image-forming composition can comprise a latent irreversible oxidation-reduction reaction composition which is capable of initiation by electron transfer from a non-silver photocatalyst. The photocatalyst can be, for example, zinc oxide or titanium dioxide. A disadvantage of the imaging material described in this patent is that the image formation is not carried out using amplification as in most heat-developable silver photographic materials. It has been desirable to overcome this problem by providing a more effective non-silver heat-developable material, that is a material which enables desired latent image amplification.
An amplification step is an important factor in increased speed imaging materials. In such materials, a catalyst is generally formed by imagewise exposure of a photosensitive material. The resulting invisible or latent image formed is then used to catalyze the reduction of a material in a high oxidation state to a visible image in a low oxidation state. In silver halide photographic materials, for example, exposure of photographic silver halide to light results in formation of silver nuclei which then catalyze the further reduction of silver halide to silver in the presence of a reducing agent.
It has also been known to produce tellurium images by disproportionation of tellurium dihalides. The images are formed in the presence of the processing liquid which helps in the disproportionation. Some unexposed tellurium dihalides, however, are dark in color causing poor image discrimination. Further, the tellurium dihalides are typically unstable in air and undergo light-induced decomposition only when moistened with an organic solvent. Accordingly, the tellurium dihalides would be expected to be disadvantageous in imaging materials designed for dry processing.
It is also known that certain tellurium (IV) compounds wherein the tellurium is bonded directly to a carbon atom can be used in imaging. In Belgian Pat. No. 786,235 issued July 31, 1972, corresponding to U.K. Specification No. 1,405,628, certain tellurium compounds wherein the tellurium is bonded directly to a carbon atom are indicated as useful in image-forming materials. The materials and process described are materials which undergo a so-called unit quantum photoreduction to yield a tellurium image. This material and process are not catalytic, that is, the tellurium (O) formed upon exposure does not catalyze the decomposition or other reaction of the tellurium (IV) compound. The process and material are, therefore, inherently photographically slow in speed and limited in usefulness.
Imaging materials have also been described wherein a substance capable of darkening when heated is heated in the presence of a catalyst such as described in U.S. Pat. No. 1,939,232 of Shepard et al, issued Dec. 12, 1933. This imaging material does not involve a heat-developable photographic material comprising an organotellurium compound. Rather it involves a combination of, for example, silver oxalate and a catalyst resulting from, for example, tellurium dichloride.
Materials are known in the imaging art in which metal nuclei are used for physical development purposes. None of these materials, however, involve nuclei in a heat-developable imaging material comprising tellurium (II) or (IV) compounds.
There has been a continuing need to provide improved tellurium containing heat-developable imaging elements, compositions and processes with physically developable metal nuclei which enable reduced concentrations of silver in the described materials. This continuing need has been especially true for non-silver heat-developable materials which enable amplification of a nuclei image without the need for use of processing solutions.